rtos: allow symbols to be optional for a particular RTOS

Default to non-optional.

Change-Id: Ifc9ddb1ab701a19c3760f95da47da6f7d412ff2e
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2355
Tested-by: jenkins
Reviewed-by: Christian Gudrian <christian.gudrian@gmx.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
rtos: add support for ChibiOS/RT 3.0

In ChibiOS/RT 3.0 the ready list pointer "rlist" is now part of the system
data structure. Since the ready list is the first element in that
structure it can be accessed via the structure's symbol "ch".

Change-Id: Idc7eaa87cb7bbad0afa0ff1dafd54283bf429766
Signed-off-by: Christian Gudrian <christian.gudrian@gmx.de>
Reviewed-on: http://openocd.zylin.com/2352
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>